In the past, a semiconductor thin film such as polycrystalline silicon (hereinafter also referred to as “poly-Si”) was widely used for thin film transistors (hereinafter also referred to as “TFT”) or solar cells. Poly-Si TFT is widely used for, for example, a switching element that constitutes a pixel circuit such as a liquid crystal display apparatus, a liquid crystal projector, or an organic EL display apparatus, or a circuit element of a liquid crystal driver using the characteristics that poly-Si TFT can be manufactured on a transparent insulating substrate such as a glass substrate due to having particularly high carrier mobility.
As a method of manufacturing a high-performance TFT on a glass substrate, a manufacturing method termed a “high-temperature process” is generally used. Among TFT-manufacturing processes, a process in which the peak temperature is a high temperature of approximately 1000° C. is termed the “high-temperature process”. The characteristics of the high-temperature process include a capability of forming a relatively favorable polycrystalline silicon film using the solid-phase growth of silicon, a capability of obtaining a favorable gate insulating layer using thermal oxidation of silicon, and a capability of forming an interface between pure polycrystalline silicon and the gate insulating layer. In the high-temperature process, a high-performance TFT having high mobility and high reliability can be stably manufactured due to the above characteristics.
On the other hand, in order to crystallize a silicon film using solid-phase growth in the high-temperature process, it is necessary to perform a thermal treatment at a temperature of approximately 600° C. for a long period of time of approximately 48 hours. The above process takes a significantly long process, and, in order to increase the throughput of the process, inevitably, there are problems in that a number of thermal treatment furnaces are required, and it is difficult to reduce the cost. Additionally, since there is no choice but to use quartz glass as the highly heat-resistant insulating substrate, the cost for a substrate is high, which does not make an increase in the size suitable.
Meanwhile, a technique for manufacturing a poly-Si TFT on a cheap large-area glass substrate by lowering the peak temperature in the process is termed a “low-temperature process”. Among TFT-manufacturing processes, a process in which a poly-Si TFT is manufactured on a relatively cheap heat-resistant glass substrate under a temperature environment in which the peak temperature is approximately 600° C. or lower is generally termed the “low-temperature process”. In the low-temperature process, a laser crystallization technique in which a silicon film is crystallized using a pulse laser having an extremely short oscillation time is widely used. Laser crystallization refers to a technique using a property of a silicon thin film in a process in which the silicon thin film on a substrate is irradiated with a high-output pulse laser ray so as to be instantly melted, and crystallized in the process of solidifying.
However, the laser crystallization technique has several large problems. One of problems is a large number of trapping levels that are locally present in a polysilicon film formed using a laser crystallization technique. Due to the presence of the trapping levels, carriers which are originally supposed to move through an active layer are trapped by application of a voltage, and thus may not contribute to electrical conduction, which results in the adverse influences of a decrease in the mobility of the TFT and an increase in the threshold voltage.
Furthermore, due to limitations of the laser output, there is another problem in that the size of the glass substrate is limited. In order to improve the throughput of the laser crystallization process, it is necessary to increase the area that can be crystallized at a time. However, since there is a limitation on the current laser output, in a case in which the crystallization technique is employed in a large-scale substrate such as a 7th generation (1800 mm×2100 mm), a long period of time is required to crystallize a single substrate.
In addition, in the laser crystallization technique, generally, a linearly-formed laser is used, and crystallization is performed by scanning with the linearly-formed laser. This line beam is shorter than the width of the substrate due to a limitation in laser output, and, in order to crystallize the entire surface of the substrate, it is necessary to scan the laser over several times. As a result, joint regions of the line beam are generated in the substrate, and there are regions which are scanned twice. These regions have significantly different crystallinity from the regions crystallized by scanning once. Therefore, element characteristics are significantly different at both regions, which causes a large variation in devices.
Finally, since a laser crystallization apparatus has a complex apparatus configuration and a large cost for consumable components, there are problems of large apparatus costs and running costs. As a result, a TFT for which a polysilicon film crystallized using the laser crystallization apparatus is used becomes an element having a high manufacturing cost.
In order to overcome the problems of the limitation on the size of the substrate and the large apparatus costs, a crystallization technique termed a “thermal plasma jet crystallization method” is being studied (for example, refer to Non Patent Document 1). Hereinafter, this technique will be described simply. A tungsten (W) anode and a water-cooled copper (Cu) cathode are disposed opposite to each other, and an arc discharge is generated between both electrodes when a DC voltage is applied. When argon gas is made to flow between the electrodes under atmospheric pressure, thermal plasma is ejected from an ejection hole opened in the copper cathode.
The thermal plasma refers to thermal equilibrium plasma, and is an ultra high-temperature heat source in which ions, electrons, neutral atoms, and the like have substantially the same temperature which is approximately 10000 K. Due to the above fact, the thermal plasma can easily heat matter to a high temperature, and an amorphous silicon (hereinafter also referred to as “a-Si”) film can be crystallized by scanning a substrate having the amorphous silicon film deposited thereon against the front surface of ultra high-temperature thermal plasma at a high rate.
As such, since the apparatus configuration is extremely simple, and crystallization is achieved under atmospheric pressure in the process, it is not necessary to cover the apparatus with an expensive member such as a sealed chamber, and a significant decrease in the apparatus costs can be expected. In addition, since utilities necessary for crystallization are argon gas, electric power, and cooling water, the crystallization technique also has a low running cost.
FIG. 16 is a schematic diagram for explaining the crystallization method of a semiconductor film in which thermal plasma is used. In FIG. 16, a thermal plasma-generating apparatus 31 is configured to have an anode 32 and a cathode 33 which is disposed opposite to the anode 32 with a predetermined distance therebetween. The anode 32 is constituted by, for example, a conductor such as tungsten. The cathode 33 is constituted by, for example, a conductor such as copper. In addition, the cathode 33 is formed to be hollow, and is configured to allow water to pass through the hollow portion so as to make cooling possible. In addition, an ejection hole (nozzle) 34 is provided in the cathode 33. When a direct (DC) voltage is applied between the anode 32 and the cathode 33, an arc discharge is generated between both electrodes. When gas such as argon gas is made to flow between the anode 32 and the cathode 33 under the atmosphere in the above state, it is possible to eject thermal plasma 35 from the ejection hole 34. Here, the “thermal plasma” refers to thermal equilibrium plasma, and is an ultra high-temperature heat source in which ions, electrons, neutral atoms, and the like have substantially the same temperature which is approximately 10000 K.
The thermal plasma can be used for a thermal treatment for crystallization of a semiconductor film. Specifically, a semiconductor film 37 (for example, an amorphous silicon film) is formed on a substrate 36, and the thermal plasma (thermal plasma jet) 35 is made to spray the semiconductor film 37. At this time, the thermal plasma 35 is made to spray the semiconductor film 37 while relatively moving along the first axis (the horizontal direction in the example shown in the drawing) that is parallel to the surface of the semiconductor film 37. That is, the thermal plasma 35 is made to spray the semiconductor film 37 while scanning in the first axial direction.
Here, the “relatively moving” means that the semiconductor film 37 (and the substrate 36 that supports the semiconductor film) and the thermal plasma 35 are made to move relatively, which includes a case in which only one of both is made to move and a case in which both are made to move. Using the scanning of the thermal plasma 35, the semiconductor film 37 is heated due to a high temperature of the thermal plasma 35, and the crystallized semiconductor film 38 (a polysilicon film in the present example) is obtained (for example, refer to Patent Document 1).
FIG. 17 is a conceptual view showing the relationship between the depth from the outermost surface and the temperature. As shown in FIG. 17, it is possible to treat only the vicinity of the surface at a high temperature by moving the thermal plasma 35 at a high rate. Since the heated regions rapidly cool after the thermal plasma 35 passes through, the vicinity of the surface remains at a high temperature for an extremely short period of time.
The thermal plasma is generally generated in dotted regions. The thermal plasma is maintained using thermionic emission from the anode 32. The thermionic emission becomes more active at locations having a high plasma density, and therefore a positive feedback is applied, and the plasma density gradually increases. That is, the arc discharge is generated intensely at one point in the anode, and the thermal plasma is generated in dotted regions.
In a case in which it is necessary to uniformly treat a tabular base material for crystallization of a semiconductor film, and the like, it is necessary to scan dotted thermal plasma across the entire base material. However, in order to build a process in which the number of times of scanning is reduced so that a treatment can be performed within a shorter period of time, it is effective to widen the irradiation region of the thermal plasma. Therefore, techniques that generate thermal plasma in a large area have thus far been being studied.
For example, a method is disclosed in which broadening gas for widening a plasma jet is ejected to the plasma jet sprayed from an external nozzle of a plasma torch from two places at the same time in a direction that intersects with the central axis line of the external nozzle so as to widen the plasma jet (for example, refer to Patent Document 2). Alternatively, a method is disclosed in which a plasma nozzle characterized by the opening portion of the nozzle path inclined at a predetermined angle with respect to the core of the nozzle path is provided, and a casing or part of the casing that constitutes the nozzle path is rotated around the longitudinal core at a high rate, thereby passing the plasma nozzle through a workpiece (for example, refer to Patent Document 3). In addition, an apparatus provided with a rotary head having at least one eccentrically disposed plasma nozzle is disclosed (for example, refer to Patent Document 4).
Meanwhile, although not aiming to treat a large area within a short period of time, a high-speed gas shield arc welding method characterized in that band-shaped electrodes are used, and disposed so that the width direction forms the welding line direction, and welding is performed is disclosed as a welding method using the thermal plasma (for example, refer to Patent Document 5).
In addition, an induction coupling-type plasma torch forming a linear thin and long shape for which a flat rectangular insulating material is used is disclosed (for example, refer to Patent Document 6).
Meanwhile, a method of generating thin and long linear plasma in which long electrodes are used is disclosed (for example, refer to Patent Document 7). Although described to generate thermal plasma, the method is to generate low-temperature plasma, and is not a configuration appropriate for a thermal treatment. If thermal plasma is generated, since the method is a capacity coupling-type in which electrodes are used, it is assumed that an arc discharge is focused at one place, and it is difficult to generate uniform thermal plasma in the longitudinal direction. Meanwhile, as a low-temperature plasma processing apparatus, an apparatus with which a plasma processing such as etching or film formation is possible by plasmatizing etching gas or chemical vapor deposition (CVD) gas is used.
In addition, a method in which long plasma is generated using a micro strip line is disclosed (for example, refer to Patent Document 8). In this configuration, since the chamber wall surface into which plasma comes into contact may not be completely cooled (not surrounded by a water cooling path), it is considered that the configuration may not work as a thermal plasma source.
In addition, an apparatus in which a plurality of discharge electrodes are arrayed linearly so as to form a linear long plasma torch (for example, refer to Patent Document 9).
Additionally, an induction coupling-type plasma apparatus having a dielectric cylinder disposed in the inside of a chamber (for example, refer to Patent Document 10) is disclosed.